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Full text of "The Cambridge Natural History"

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Horse. The fact that it is occasionally met with in Man is an
additional proof of the, in many respects, ancient structure of the
highest type of Primate.

The radius and the ulna, which together constitute the fore-
are both present in a large number of mammals, but the
tends to vanish in the purely walking and digitigrade
Ungulates, being present, however, in the
more ancient forms of these Ungulates.
In Man and in many other mammals
the radius can be moved from its normal
position and crossed over the ulna; this
movement of pronation has been per-
manently fixed in the Elephant, where
the bones are crossed but cannot be altered
in position by the contractions of any
muscles. Other types agree with the
Elephant in this fixation of the two bones.
The bones of the wrist show great
variation among mammals. The greatest
number present are to be seen in such a
type as the Mole. Here we have a
proximal row, consisting of the scaphoid,
lunar, cuneiform, and pisiform, which are
-Bones of fore-arm arranged in their proper order, beginning
~" "*~ with that on the radial side of the limb,
that side which bears the first digit. A
second row articulates proximally with
these bonelets and distally with the meta-
carpals ; the bones composing it are, men-
tioning them in the same order, trapezium,
trapezoid, centrale, magnum, unciform.
The centrale does not, however, really belong to the distal
carpal row, and is as a rule situated in the middle of the carpus
away from articulation with the metacarpals. It is a bone which
is not commonly present in the mammalian hand, but is present
in various lower forms, such as the Beaver and Hyrax, It also
occurs in such high types as the majority of Monkeys; it is to
be found in the Human foetal carpus. Many extinct forms
possessed a separate centrale. Its importance in the formation of
the interlocking condition of the Ungulate foot is referred to later,

Fro.  81.-


form; ce, centrale; t, lunar ;
m, magnum j p, pisiform ;
R, radius ; rs, radial sesa-
rnoid (falciform) ; , sca-
phoid ; tdt trapezoid;
trapezium ; 17, ulna ; u,
unciform ; i~v, the digits.
(From Flower's Osteology.)